IRON-SULFIDE CONCRETIONS OF THE OHIO SHALE: GLIMPSES OF DEEP SUBSEAFLOOR MICROBIAL ECOSYSTEMS OF THE LATE DEVONIAN

Recent interest in black Devonian shales for natural gas exploitation has brought shale research into new importance. At the same time, advances in the understanding of deep subseafloor microbial ecosystems have revealed that microbes play a more pivotal role in geologic processes than ever imagined. The convergence of these two areas provides an excellent opportunity to reexamine enigmatic concretions of the Ohio Shale. The Huron member of the Ohio Shale is distinguished by the presence of carbonate concretions. These concretions vary considerably in size and differ in composition from the host rock. Their origins have been questioned since they were first described in 1873. Current models attribute concretion formation to abiotic mineral replacement of organic substances. However, smaller, iron-sulfide concretions inhabit the same horizons and are less well studied. Iron-sulfide mineralization has been attributed to biotic processes and may be connected to the presence of the larger carbonate concretions.

Iron-sulfide concretions have been collected from three sites in Delaware and Franklin Counties in Ohio. These samples were investigated using reflected light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. Based on morphology, composition and size, the iron-sulfide concretions have been categorized into four different stages of paragenesis. These stages correlate well with known microbial zonations of deep subseafloor environments.

Understanding the microbiological processes at work in the creation of black Devonian shales may shed light on how to exploit modern microbial systems to develop sustainable sources for carbon fuels in the future.